This invention relates generally to a ballast for a high intensity discharge (HID) lamp, and more particularly to an ignition scheme for an HID lamp.
In order to ignite a conventional HID lamp, the nominal U.S. utility line voltage of 120 volts AC (VAC) is increased and in combination with very high amplitude voltage pulses of short duration is applied to the lamp. A ballast can step up the nominal U.S. utility line voltage by magnetic transformation. Such transformation results in a ballast which is undesirably bulkier and/or having unacceptably higher losses.
When the nominal line voltage is sufficiently high (e.g. at about 200 volts RMS or more), transformation is not required. A conventional ballast, without such transformation, is disclosed in U.S. Pat. No. 4,461,982. Such ballasts in providing high amplitude voltage pulses of short duration include both an oscillator and an ignitor. The amount of energy supplied by the oscillator to the lamp, however, is limited. Transfer of energy from the oscillator to the lamp occurs only during operation of the ignitor, that is, for the duration of each ignition pulse. The amount of energy stored by the oscillator and therefore available to the lamp is also limited by a storage scheme which imposes sharing of energy to be stored between the oscillator and an ignitor.
Accordingly, it is desirable to provide an improved HID ballast which is less bulky and more energy efficient. The improved ballast in providing energy from the boosted nominal line voltage to the HID lamp during starting should not be limited to the duration of the one or more ignition pulses or be limited in the amount of energy supplied to the lamp due to sharing of energy storage with an ignitor.